For a number of paper applications the physical parameter of wet strength is of fundamental importance, examples being that of papers of tissue quality, as well as in e.g. paper-bags and paper-sacks.
The Technical Association of Pulp and Paper Industries (TAPPI) defines the wet strength of a paper as the tensile strength of the paper after it has been completely soaked with water. An untreated paper keeps only 2-8% of its original dry strength when soaked with water. However, the wet strength of a paper can be improved by the addition of wet strength enhancing additives to the pulp, making it possible to retain as much as 25-40% of the dry tensile strength in the wet paper.
Paper obtains its strength from the interfibrillar hydrogen bonds which are created when the cellulose fibres are drawn together by surface tension during the drying process. When the paper is re-wetted, the fibres swell and the hydrogen bonds are loosened up and destroyed. The paper looses its strength. Wet strength agents help to retain some of the strength, up to 40-45% of the dry strength, when the paper is fully soaked in water.
The true mechanism behind wet strength is not yet fully understood and there is a possibility that different agents may give wet strength by different mechanisms.
Chemicals giving wet strength are all water-soluble synthetic polymers, prepolymers, with functional groups. They have the possibility to react further, to crosslink and increase the molecular weight during the drying process. They are all added to the stock as solutions in water or as colloids (mainly melamine resins). They are cationic by charge and are retained by their charge and molecular weight mainly on the fines particles. The anionic charge is much stronger and the surface area is many times larger of those fines than of the coarse fibres.
Three main mechanisms can be postulated for the wet strength:
The first postulate suggests that resin and fines particles containing a lot of resin are accumulated at the fibre-fibre crossing by the forces produced during the drying process. The resin crosslinks with itself, homo crosslinking, in the mixture. A composite material of resin and cellulose is formed. Those covering layers, on both sides of the fibre, protect against water and the fibres are kept in place and cannot swell. The hydrogen bonding between the fibres are more or less undisturbed and the strength is retained. PA1 The second postulate suggests that when the paper treated with wet strength resin, is heated the resin is cured. The crosslinked polymer forms a net work on the surface of the coarse fibres which protects the existing fibre to fibre bond, making them resistant to water. This protection maintains the strength of the paper by reducing the swelling in the bonding areas. PA1 The third postulate suggests that there is a reaction between the resin and the cellulose fibres. Such chemical bonding would result in the cellulose fibres having a reduced swelling capacity in water which would lead to less breaking of the interfibrillar bonds. PA1 The first stage is the methylolisation of urea by formaldehyde in a molar ratio of slightly above two formaldehyde to one urea. Some cationic amines are added to the mixture where they react with formaldehyde and are incorporated in the resin via methylol groups. PA1 The second stage involves condensation polymerisation between the methylol end groups, promoted by high temperature and low pH's, resulting in ether or methylene linkages, with the elimination of a water molecule or a formaldehyde molecule.
Wet strength resins were developed rather late in the history of paper making. It was not until the 1940s that different urea-formaldehyde (and melamine formaldehyde) formulations were tested as wet strength agents and they remained being commercially important wet strength resins since then. Improvements have been made and other polymers have been developed during the years.
Urea-formaldehyde, UF, resins are low in cost, they are easy to repulp and are not sensitive to interference with dissolved substances in the paper making system. Two stages are involved when making urea-formaldehyde resins.
The polymerisation process is interrupted at a certain molecular weight by lowering the temperature and changing the pH above 7. The resins are delivered to the paper mill as solutions with dry contents around 40% and with the ability to crosslink further. Urea-formaldehyde resins do not need pre-treatment at the paper mills. A pH of 4,5 in the white water system is the optimum for the use of urea-formaldehyde. It is also a well known fact that urea formaldehyde resins need both time and temperature to crosslink, cure, totally. That is why only half of the possible final wet strength usually is achieved off machine. Full wet strength is then reach after two or three weeks depending on how the paper reels are stored. The resins can be added as delivered at almost any point in the system. Typically resin addition levels vary between 0,5 to 3,0 % depending on the type of paper products desired.
The mechanism behind the efficiency of urea formaldehyde resin is either the first or the second postulate defined at p. 2 or both.
The resin has been fully investigated and the methylol groups are not able to react with hydroxyl groups on fibres and fines. The crosslinking occurs only within the resin itself, homo crosslinking. The awareness of environmental problems has led to new urea formaldehyde resins with low emission of formaldehyde.
Both urea and melamine resins require acid conditions to cure effectively in the paper. Alkaline-curing amine epichlorohydrin condensation products, PAAE, related to epoxy resin adhesives, were invented in the 1950s. They gained immediate acceptance and started to replace urea formaldehyde in many formulations where pH could be increased. They offered high cost-effectiveness, compatibility with alkaline sizes, reduced machine corrosion, and other benefits.
The most common polymeric amine-epichlorohydrine resin is prepared in two steps. At first the polymer backbone is produced from a dibasic acid such as adipic acid which is heated together with diethylenetriamine forming a linear polyamineamide by condensation. The second step involves a reaction between epichlorohydrin and all the secondary amine groups in the backbone. The chlorohydrine groups formed by the epichlorohydrin are transformed to reactive azetidiniumgroups which react with the tertiary amino groups in the backbone forming a rather crosslinked, still water soluble, net work. This crosslinking is interrupted at a certain molecular weight by lowering the temperature and changing the pH below 5. The resin is delivered to the paper mill as a solution with a dry content around 20% and with the ability to crosslink further. The resin is added to the stock and retained mainly by the fines.
The cured resin is not easy to hydrolyse. Therefore the wet strength imparted by azetidinium-type resins is permanent rather than fugitive and proper attention to special techniques is required for successful broke reworking. The awareness of environmental problems has led to new PAAE resins with very low content of chlorinated byproducts, which could give contribution to AOX (Adsorbable Organic Halogenates).
The UF and PAAE resins are dominating the market for wet strength but there are some other polymers used mainly for specific paper products. Glyoxylated polyacrylamide (gPAM) is specifically used in wet strength towelling. gPAM is prepared by crosslinking of a low molecular weight PAM with glyoxal. The PAM is normally prepared with a cationic co-monomer of a quaternary type to provide good retention to the fibres or fines. gPAM is supposed to impart wet strength to paper primarily through covalent bond formation between resin and fibre. The bonding is formed by reaction between a hydroxyl group from the cellulose and an aldehyde group from the resin. These bonds are formed where gPAM is adsorbed onto the fibre surfaces within the fibre-to-fibre bonded area. Drying conditions are said to favour formation of an extensive amount of covalent bonding between the resin net work and each of at least two contacting fibres.
The method of measuring the wet strength presently used in the paper production industry is also defined by TAPPI. According to this method, the wet strength, in terms of e.g. the wet tensile index, in kNm/kg, is mechanically measured in the standard test method T 456 m-49 using a standardized instrument, a tensile tester. Briefly, this quite straightforward method consists of soaking a paper sample in water and measuring the tensile strength of the thus treated paper in the tensile tester.
When testing the wet strength of a paper in a paper production plant, the paper sample is taken at the end of the paper machine, from the last roll, called the reeling drum. Only when the reeling drum is full, containing a large quantity of paper, e.g. 20 tonnes the sampling is performed.
Before testing the wet strength, the paper sample must be submitted to an accelerated curing. This curing step is necessary in order to obtain a relevant measure of the wet strength of the paper since the paper on the roll is also naturally subject to a certain curing, whereby the wet strength is somewhat increased.
One major drawback with this way of monitoring the wet strength is the delay between a change in a parameter at the manufacturing of a paper and the answer of the determination of the wet strength of that paper. This delay may lead to important losses in case the wet strength proves to be inadequate since, by the time this assessment has been accomplished, there may be very large quantities of paper of this inadequate wet strength produced.
It is obvious that the method of testing the wet strength presently in use in the paper production industry is a drawback to the productivity and the economy of the paper production process. Thus, there is a definite need for a more convenient method of testing the wet strength in the paper manufacturing industry.
The present invention has for an object to offer a solution to said problem, by providing a method that allows the monitoring of the wet strength of the paper during the production process. This object is attained by the combined use of spectrometric and chemometric techniques.
According to the invention, the paper or the pulp in or from the production line is submitted to spectrometric analysis. However, the pulp as well as the paper represents a multi-component system or a system having a high degree of background interferences which increases the problems of spectrometric analysis.
The use of multivariate data analysis in the characterization of multi-component systems is presently a field of development. Applied generally to the field of chemistry, and particularly to the field of analytical chemistry, those several statistical methods also are termed chemometric methods, forming the discipline of chemometrics. The technique of chemometrics is more fully explained in S. D. Brown, "Chemometrics", Anal. Chem. 62, 84R-101R (1990), which by reference is incorporated herein in its entirety.
An example of the use of chemometrics is given in the thesis of Wallbacks (Pulp characterization using spectroscopy and multivariate data analysis, L. Wallbacks, Dept. of Organic Chemistry, Univ. of Umea, Sweden (1991)), who has shown that multivariate data analysis can be used to predict various physical properties as a function of the initial characteristics of the unbeaten pulp and the effect of beating.
Further, Brown et al, in the U.S. Pat. No. 5,121,337 (1990) disclose a method, based on multivariate data analysis, for correcting spectral data for data due to the spectral measurement process itself and estimating unknown property and/or composition data of a sample using such method.
On the other hand, Richardson et al, in U.S. Pat. No. 5,242,602 disclose a method for simultaneously measuring the concentration of multiple chemical components, which they call performance indicators, in an aqueous system, by the analysis of the spectrum of the aqueous system in the wavelength range 200 to 2500 nm and by applying chemometric algorithms to the spectrum to simultaneously determine the concentrations of the different performance indicators.
Weyer, U.S. Pat. No. 5,104,485 discloses a method for measuring extremely low concentrations of non-aqueous constituents or chemicals in a water/matrix, including differentiating between pulp fines and extremely low concentrations of individual chemicals in a water/cellulose matrix such as occur in papermaking. The water/matrix is exposed to the near-infrared spectrum from 1000 to 2500 nm to produce a record voltage that is directly proportional to the absorption by the non-aqueous constituent. The amount non-aqueous constituent is determined from voltage values of incremental additions of the non-aqueous constituent.
In addition Hercules reported in a research disclosure (December 1992/945) that in the papermaking process, a water/cellulose mixture is laid on a wire screen and the water is filtered off leaving the fibers and various additives. The paper sheet produced is composed of cellulose fibers, fillers such as clay and calcium carbonate, and additives such as optical brighteners, sizes, and wet and dry strength resins. Various instrumental systems are available for measuring some of these constituents such as the clay. These systems, however, are limited in the determinations that can be carried out. A method for determining several individual chemical constituents simultaneously in a paper sheet has been developed according to Hercules. Radiation from a near infrared sourse is allowed to impinge upon the paper sheet, and after interaction of the radiation with the chemical constituents in the sheet, the reflected radiation is collected and stored. The chemical composition is calculated from the stored data after mathematical treatments are applied. The measurement system is calibrated via samples of known composition. Use of the full near infrared spectrum from 1100 to 2500 nanometers permits the analysis of several constituents simultaneously, especially when derivatives are employed as part of the mathematical treatment. This analysis aids in determining the extent of retention of the chemical additives and fillers.
However, the present inventors have shown that four steps should be involved for a useful quantification of a chemical on the basis of spectroscopy. The first step is recording the determination of the emission, transmittance or reflectance values from a huge number of wavelengths (e.g. 300 to 600 numbers of wavelength are not uncommon). The second step is data processing the spectral data, which is essential in the NIR region (800-2400 nm). The third step is transformation of data, usually by centring, normalisation or autoscaling the data. The forth step is to find the mathematical expression for the calibration function (data analysis).
The description of the method according to Hercules only disclose the first and second step. The spectral information is collected, followed by an undefined mathematical treatment. The only detail that is given is the application of derivatives (which is a commonly used technique within spectroscopy). Nothing is revealed about the numerical algorithm used for the transformation of data and algorithm for calibration. These steps are of utmost importance to obtain a useful quantification of a chemical or a property on the basis of spectroscopy.
However, according to this invention specific algorithms have been applied to overcome especially two disadvantages, namely:
1. The number of wavelengths can be considerable and outnumbers the number of samples, used for the calibration. As an example, if the reflectance of 300 wavelengths are recorded for 20 samples, with conventional mathematical models only the values from the number of samples minus 2 can be used for the calibration. Thus, in this case only values from 20-2=18 wavelengths can be used and the information from the other 282 wavelengths cannot be taken into account. According to this invention all spectral information can be used and compiled by transferring all the information recorded into so called latent variables based on principal component analysis.
2. The spectral information is often highly correlated which seriously affect the success for quantification. If the spectral information is transferred into latent variables by principal component analysis a higher degree of orthogonalisation is obtained which can be a crucial factor for success.
Moreover, none of the above mentioned authors suggests how to solve the problem of determining the wet strength present in a paper in a paper production process in a way permitting the monitoring of these parameters and furthermore no details of the calibration procedures are given. It should be emphazised that the expression "determination" in this context can be interpreted either as a qualitative analysis or as a quantitative analysis. A qualitative analysis is the determination of the presence of a chemical or property while quantitativ analysis relates to the estimation of a certain value, including the degree of uncertainty of this value (expressed in statistical terms such as confidence interval etc.). The object of the present invention is to provide a reliable and precise way of monitoring--i.e quantification--the wet strength present in a paper by spectroscopic measurement in combination with multivariate data analysis using chemometrical techniques.
None of the above mentioned authors suggests how to solve the problem of determining the wet strength of a paper in a paper production process in a way permitting the monitoring of this parameter. This, however, is the object of the present invention, which provides a reliable and precise way of monitoring the wet strength of a paper by a rapid chemical analysis coupled with multivariate data analysis using the technique of chemometrics.
The object of the invention thus is to provide a method of determination of wet strength of paper in pulp and paper in real time without the use of the traditional lengthy mechanical measurements.
It is another object of the invention to provide a method of maintaining an effective process control programme wherein the wet strength of a paper is monitored to detect any change and provide control input, assuring optimum dosage levels for wet strength enhancing chemical additives and mechanical treatment of the pulp.
The methods and means as disclosed according to the invention are those as further defined in the claims.